1. Field of the Invention
The present invention relates to a lead-free current fuse, which is suitable for electronic equipment and electronic components, for example, and a method of making the current fuse.
2. Description of the Related Art
Various kinds of compact fuses, which are surface-mountable on a printed circuit board or the like, have been proposed. For example, a fuse in which a fuse element stretches in a hollow space within a rectangular case formed by attaching a ceramic casing main body and a lid has been disclosed in Japanese Patent Application Laid-Open No. 8-222117.
In the case of this compact fuse, a fuse element and tin-lead solder, which has been adhered to the interior of metallic cap in advance, are soldered.
On the other hand, tin-lead solder is essential for fabricating or assembling electronic equipment; however, it contains lead which is harmful to the human body and the like. Therefore, solder not containing harmful lead is desired by electronic equipment industries or related industries, and various kinds of lead-free solder have been proposed. For example, a lead-free solder alloy containing 7 to 10 percent by weight of zinc (Zn), 0.01 to 1 percent by weight of nickel (Ni), and the balance consisting of tin (Sn) has been described in Japanese Patent Application Laid-Open No. 9-94688.
On the other hand, a solder material that has been disclosed in Japanese Patent Application Laid-Open No. 2000-15478 is developed considering environmental issues after discarding electronic equipment, and is a lead-free solder material containing tin, 3 to 18 percent by weight of zinc in proportion to that tin and a small amount of additives (e.g., nickel).
However, when solder containing 10 percent by weight of tin and 90 percent by weight of lead is used to manufacture the fuse described in Japanese Patent Application Laid-Open No. 8-222117, a ceramic substrate absorbs heat generated when that solder melts. Therefore, a temperature to heat metallic caps must be set at approximately 400 degrees C., which is approximately 100 degrees C. higher than the melting point of solder.
Such temperature degrades plating on the surface of metallic caps, resulting in deterioration of solderability when mounting a fuse on a substrate.
On the other hand, the above-described lead-free tin-zinc-nickel solder is used for mounting a fuse on a substrate. That solder used in the interior of component causes a problem of deterioration of quality, such as break of a fuse element when the solder within each component melts in a reflow process when mounting the fuse on the substrate.